The objective of the project is to record single cell activity from visual cortex in awake monkeys while they perform a brightness discrimination task in which one of the stimuli is masked by metacontrast. In metacontrast, the apparent brightness of a briefly flashed stimulus is reduced if it is followed by another flashed stimulus having contours in common with the first. This phenomenon is a promising tool for elucidating the coding of visual stimuli because it separates stimulus intensity from perceived brightness. The hypothesis to be tested is that early components of the single-cell visual evoked response carry information adequate for stimulus detection, while only the later components carry information about attributes such as brightness and form. The study will also measure metacontrast psychophysically in the monkey for the first time.